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Critical Measurement of Well Thermal Properties to Support Design A.A. Koenig, Ph.D. ARB/Geowell NGWA Geothermal Forum Dec 10, 2009 New Orleans

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Abstract For large tonnage (100 tons or greater) HVAC systems, it is imperative that a test well be drilled and that measurement of the thermal characteristics of the well (as representative) be undertaken prior to a final design. The defining thermal properties include: the “effective” thermal conductivity, K, of the formation surrounding the bore; the thermal diffusivity, α, and the characteristic thermal decay rate, τ. Armed with this knowledge, the geothermal engineer can begin to specify the design (wetted ft per delivered ton) to meet a building heating or cooling load. This paper will demonstrate the measurement, analysis and interpretation required to extract this information, using a number of geothermal installations to exemplify the approach.

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A Typical SCW Bore Cross-Section 10”-12” bore with 60’-120’ of 8” steel casing grouted into competent bedrock 8” open bore to 150’-200’ 6” open bore thereafter to final depth Final depth range: 800’-1500’ (deeper for urban constrained areas) Bore can encounter faults, etc. which render the borehole unstable and subject to collapse

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SCW Design Considerations How do I recognize a good site opportunity to reduce installed capital cost by specifying open loop SCW design? How deep should I go in a single bore? What should I do if I encounter significant water while drilling? How do I specify how many wells and at what depth to meet a given building load?

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Response Ideal: fractured hard rock with high effective thermal conductivity (k ≥ 2 BTUH/ft°F); depth to bedrock & water < 70’ Drilled depth should be left somewhat flexible to allow for termination when large amounts of water preclude economic drilling or there is potential for approaching a salt water interface What is important is the total wetted feet of bore supported by thermal & hydro-geologic testing followed by simulation.

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Map of Where We Are Going Measurement needs Analysis of measurement to yield critical thermal & hydro-geologic properties that dictate SCW design What controls the limits of specified borehole depth? How many wells do I need to serve a given load, and how should I specify this? A look at some examples.

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What Size Project Commands Testing? Set aside 5% project cost for testing With a 2 day thermal test charge of $15,000 per well, the minimum size installation is then $300,000, or approx. 100 tons Additional wells should be drilled and tested for larger tonnage projects; consider the extremes of the well field layout The average measured borehole thermal values should be used to support the design.

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Purpose of Thermal Testing Measure the thermal properties of the wellbore rock in order to: support an informed decision on continued drilling (total no. drilled feet) develop simulations of the geothermal seasonal system performance incorporate the building hr x hr block load expectations utilize information on the hydrogeology (interconnectedness of the wells).

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SCW Thermal Test Layout Why a propane source? Take the example of a 1000’ well designed to accept 80 ft/ton => 12.5 tons or 150 kBTUH; with a 84% combustion efficiency, the heater will need a heat source of 180 kBTUH for the test duration of at least a day.

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Water Heater Selection Features: 84% efficient pool/spa heater rated: 150-250 kBTUH Propane (100 gal) or NG Max temperature 105°F Electronic set: temperature limits and rise Cuper-Nickel HX Portability (120 lbs) 2” ports Designed for outdoor use Well insulated

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Test Basis 1-2 day continuous test period @ 150-250 kBTUH, depending on heater selection You’ll need 50-100 gal. propane (expect to get 32 hrs out of a 100 gal tank for the larger 250 kBTUH heater) 50 gpm test flowrate => 8°F ΔT (spec) Followed by 14 hrs cool down Avg. water temperatures and flowrate tabulated every minute (3000 data pts) Typical SCW water temperature rise from 53°F to 87°F depending on the rock conductivity & water encountered.

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Comparison of Thermal Testing Results with Expectations Based on the Carslaw & Jaeger Model The thermal test results over 65 hrs. shows good agreement with the C-J model. Note that a typical 48 hr duration overlays the rising portion of the curve and falls short in supporting prediction of seasonal SCW performance, i.e. the flatter portion. It would be beneficial to continue the testing, but this becomes an expense tradeoff.

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Thermal Test Equipment Setup

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Test Equipment Layout

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Labview + NI FieldPoint DAQ

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Detailed DAQ Measurements Q° = 209,275 BTUH, L(wetted) = 1460’ => 83.7 ft/ton

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Analytical Fit to Test Data: (T av -T ∞ ) = 4.30 LN(t) + 6 Slope=4.30 k(eff) = 2.65BTUH/ft°F q ° = 22.8 BTUH/ft K eff = q ° /(2*Slope) = 2.65 BTUH/ft°F Slope = 4.30 Intercept = 6.01

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Data Analysis Using Graphical Solutions for k & α K-value: measured from slope of data ΔT avg vs. LN(t) fixed heat flux: q ° = Q ° (BTUH)/(2 πL) = 22.8 BTUH/ft measured slope, s = 4.304 K = q ° /(2s) = 22.8/(2*4.304) = 2.65 BTUH/ft°F Thermal diffusivity, α (sqf/hr): measured using the intercept value (ΔT o ) @ LN(t)=0: α = {r o /δ o EXP[ΔT o /(2s) ] } 2 measured intercept, ΔT o = 6.013 α =.068 sqf/hr (1.62 sqf/day) or averaged from individual datum using: α = {r o /2 EXP[ ½( ΔT i /slope-LN(t)) ]} 2 Check of α: (ρC p ) = k/α = 2.65/.068 = 39.2 BTU/cuf°F Is this reasonable? Can it be verified from independent analysis of rock samples taken at various depths during drilling?

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Contribution of Mobile Water at the Bore Fracture Face to K(eff) K eff = Q°/(2A bore S 2 ) where S=slope K rock = Q°/(2A bore S 1 ) =(Q°- q x )/(2A bore S 2 ) K rock = K eff (1- q x /Q°), or K eff = K rock /(1- q x /Q°) qxqx

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Thermal Relaxation Study Why is this measurement important? Temperature decay rate is important to the understanding of what temperatures will be achieved as the building load comes off and the geothermal wells are allowed to relax. This time constant goes into a model simulating well performance with time. Two ways to measure borehole thermal relaxation rate: remove heat load; measure & record SCW temperatures over 14 hours of cool-down by either allowing wellbore water to circulate while measuring temperature exiting the well, or affix temperature probes to the PVC separator at select depths (e.g. top, mid, bottom) record without circulation.

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Thermal Relaxation Discrete Data

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Analytical Form for Relaxation T(t) = T ∞ + (T start -T ∞ ) EXP(-t/τ) where T(t) is SCW temp. @ time, t(hr) T ∞ is the undisturbed (ambient) ground temp. T start is the water temp. @ start of relaxation τ is the time constant (hr) for thermal relaxation. Note: LN[( T - T ∞ )/( T start - T ∞ )] vs. t is a straight line with slope = τ.

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Thermal Relaxation Study

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Thermal Relaxation Analysis (Early Portion) τ =7 hrs

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Thermal Relaxation Analysis (Later Portion) Time Constant: τ ≈ 20 hrs

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Alternative Form: Semi-log Fit to Relaxation Data

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Another Example: Thermal Relaxation Study

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A Comparative Look at Thermal Characteristics Measured at Various Sites SiteRock TypeK( BTUH/ft°F ) effective α (sqf/day ) effective r (thermal recovery rate ) S-F Schools Royersford, PA red shale with large yield ≈ 250 gpm 2.982.354.06 10.6 (top) Friends Center Philadelphia Wissahickon schist with 100 gpm yield 2.651.653.48 Villanova University gneiss with few fractures & 5 gpm yield 1.861.037.375 99 th ACE Londonderry, NH gneiss with 50 gpm yield; num fractures > 200’ 3.092.106.10

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General Observations Regarding Thermal Properties High K values (>2 BTUH/ft°F) are likely due to the presence of significant amounts of mobile ground water present at the bore, which increases the effective conductivity. In general, ground water infiltration in the bore enhances the effective K-value in a linear contribution: K eff K(rock) + F (G f ), where G f is the equiv. total infiltrating flow (gpm) over the face of the bore, as it is modeled.

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How do you use this information to develop a preliminary SCW design? Well bore measurements (k, α, τ)

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Wellfield-HP Interface

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Heating & Cooling Load Considerations 1.Is there a balanced heating & cooling load for this site? Typically, the answer is NO, which means that the designer is forced to make a choice to: a.Meet only the smaller of the two loads using geothermal, in which case, the additional burden must be supplemented with a conventional HVAC system, or b.Over-design the system to meet the most demanding load. 2.The most economical solution to (1) is (1a) assuming that the additional first cost of implementing (1b) is an impediment to proceeding with the project. Note also, that (1a) presents an annual balanced load to the ground.

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How to Design a SCW Field to Meet a Load 1.Start with the building design LOAD. Is this heating (or cooling) dominated? a.If heating dominated, the SCW design must be cognizant of preventing a potential freezing condition in the well as water is returned cooler with each circulation b.If cooling dominated, the SCW field is burdened additionally by having to reject the heat of compression from the heat pumps. This amounts to an additional 20%. 2.Use the thermal test results specific to the site to guide the design: a.If bleed is acceptable, then you will need to determine an optimal design (capital cost & annual performance) that will minimize the design (ft/ton) at an acceptable bleed percentage, e.g. 10% for “x” hours a year b.If a NO-bleed design is contemplated, you need to select a conservative design (ft/ton) that is consistent with the measured effective thermal conductivity.

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No-Bleed Design Guidance

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No-Bleed Design Case Example Let’s assume that one measures & determines a representative K-value = 1.9 BTUH/ft°F Enter the curve at the bottom at K=1.9 and read across to the y-axis to arrive at a conservative design basis of 90 ft/ton For example, to meet a building load of 258,000 BTUH (215 tons heating equiv.), one will require a total wetted bore length of: 215 x 90 = 19,350 ft of bore Assuming that the ($/ft) risk of drilling is the same down to depths of around 1300’, and static levels are around 40’, then 15 wells will be required, drilled to an average depth of 1330’ (1290’ + 40’). If significant water is encountered during drilling a well, then one will need to re-visit in-situ the design depth for a better economic selection.

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Ground Water Flow Coupling to SCW Model Study of GW Flow Impact on k(eff)

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Two Contributions to k(eff) 1.Heat transport of “thermal” water from the bore by advective flow, along with 2.Influx of ground water due to the natural hydraulic gradient [Note: as the rock & water around the borehole store heat, the avg. temperature rises in time: T(SCW) => T(avg)]

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Base Case: K(eff) @ Zero GW Flow

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GW Flow Modification to Model Model of 48 hour thermal test (continuous heating): (1)K(eff) increases with ground water flow rate (2)Non-linear behavior for t > 1 day

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K(eff) Increase with GW Flow Friends Ctr Data

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Modeling & Simulation Three Elements: hr x hr daily load representation for each month; heat pump (EWT) characteristics, and SCW dynamic thermal model

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Model of Daily Heating & Thermal Relaxation Developed from Testing

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Translation from Building HVAC Daily Peak to Avg. Daily Heating/Cooling Rate

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Daily Avg. Load Interpretation

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Heat Pump Duty Cycle Approximation

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30 Ton w-w Heat Pump Characteristics

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Daily Cooling Load Simulation

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One Week Cooling Results

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One Month Cooling Results

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Cooling Season (4 mo.) Results

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Conclusion Measurement of the well bore thermal properties is critical to support a proper design of SCW geothermal field to meet a building load Modeling & simulation of the HVAC system operation allows the designer to fine-tune the geothermal design For more information, we recommend visiting HeatSpring Institute (www.heatspring.com) for their course offering on SCW Design.www.heatspring.com

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